The present invention relates to a tin-free steel (TFS) having a first layer, of metallic chromium, on a steel base, and a second layer, of hydrated chromium oxide, on the first layer, which can be used for a nylon-adhered can body requiring excellent lacquer adhesion after aging in hot water and under retort conditions.
Recently, lacquered TFS, rather than electrotinplates, has largely been used for manufacturing carbonated beverage cans and beer cans, since it exhibits lacquer adhesion which is superior to that exhibited by electrotinplates.
The ordinary metal can consists of the two can ends and a can body. In the case of lacquered TFS, the seaming of the can body is mainly carried out with nylon adhesive by using the Toyo Seam and Mira Seam methods. In these cases, the nylon adhesive is inserted not between the plain TFS surfaces, but between the lacquered TFS surfaces. An epoxy-phenolic type of lacquer is generally applied to the TFS. Therefore, the bonding strength of the adhered part of the lacquered TFS can body is shown by the weakest bonding strength among three interfaces: (1) between the metallic chromium layer and the hydrated chromium oxide layer, (2) between the hydrated chromium oxide layer and the lacquer film, and (3) between the lacquer film and the nylon adhesive. The nylon adhered part of the lacquered TFS can body not only has an acceptable bonding strength in a normal state, i.e. at room temperature and atmospheric pressure, but also a bonding strength which can satisfactorily withstand internal pressure caused by the contents of the can, such as beer and carbonated beverages.
However, when a can having a TFS can body seamed by nylon adhesive after lacquering is used as a container for foods such as fruit juices, which are immediately hot-packed after pasteurization at a temperature of 90.degree.-100.degree. C., or as a container for foods such as coffee, meat and fish, which are pasteurized by hot steam at a temperature above 100.degree. C. in a retort after being packed in the can at about 100.degree. C., the lacquer film may be peeled off from the TFS surface. Thus, a drop in the degree of vacuum in the can may occur due to partial loss of adhesion between the adhered parts of the can body, because the lacquer adhesion of conventional TFS becomes poor after aging in hot water and under retort conditions. Therefore, it is not possible to use conventional TFS cans seamed with nylon adhesive after lacquering, for pasteurizing the contents of the cans packed at high temperatures.
It is assumed that the deterioration of the lacquer adhesion of conventional TFS, after aging in hot water and under retort conditions, depends on the properties of the hydrated chromium oxide in the TFS.
In general, there are two well-known types of manufacturing processes for the production of commercial TFS. The first type is a one-step process in which metallic chromium and hydrated chromium oxide are formed in one operation by using one electrolyte composition. The second type is a two-step process in which first metallic chromium is formed by using one electrolyte composition as a chromium plating solution, and then hydrated chromium oxide is formed on the metallic chromium layer by using another electrolyte composition. In both types of processes, addition agents such as sulfuric acid and fluoride are added to the electrolyte compositions in amounts which result in incorporation of substantial amounts of sulfur and/or fluorine into the hydrated chromium oxide layer.